Semiconducting chalcogenide films are typically used as absorber layers in photovoltaic devices, such as solar cells. A chalcogenide is a chemical compound consisting of at least one chalcogen ion (group 16 (VI) elements in the periodic table, e.g., sulfur (S), selenium (Se), and tellurium (Te)) and at least one more electropositive element. As those of skill in the art will appreciate, references to chalcogenides are generally made in reference to sulfides, selenides, and tellurides only. Thin film based solar cell devices may utilize these chalcogenide semiconductor materials as the absorber layer(s) as is or, alternately, in the form of an alloy with other elements or even compounds like oxides, nitrides and carbides, among others. Chalcogenide (both single and mixed) semiconductors have optical band gaps well within the terrestrial solar spectrum, and hence, can be used as photon absorbers in thin film based solar cells to generate electron hole pairs and convert light energy to usable electrical energy.
Physical vapor deposition (PVD) based processes, and particularly sputter based deposition processes, have conventionally been utilized for high volume manufacturing of such thin film layers with high throughput and yield. These thin film layers can be deposited by the sputtering (in the form of reactive/non-reactive or co-sputtering) of high purity sputter targets. Generally, the quality of the resultant semiconductor thin films depends on the quality of the sputter target supplying the material which, similarly, generally depends on the quality of the target's fabrication. Providing manufacturing simplicity while ensuring exact stoichiometry control can ideally be achieved by non-reactive sputter of high purity sputter targets of the appropriate materials having the same stoichiometry. However, as some of these materials have different atomic specie with varying sputter rates, as well as different melting points, achieving the exact desired stoichiometry in the thin film presents a challenge. Any non-stoichiometry in the resultant thin film can contribute to non-adjusted charge compensations in the structure and can affect the device characteristics. Additionally, the incorporation of impurities from the sputter targets into the thin film absorber layers can also cause inconsistent and unreliable device characteristics. By way of example, impurities can act as trap levels (which would vary based on different impurities and their relative concentrations) in the band gap. Furthermore, the sputter targets themselves should have adequate density in order to minimize arcing and defect generation during the deposition process, as these can limit the process yield.